Political diaries are usually an abomination. They amount to the betrayal of friends and colleagues, written for self-glorification, sold for money and lightly disguised as contributions to history. However, I gladly admit that Chris Mullin's Decline and Fall, the second volume of his journal of life as an MP, is different from the usual outpouring of self-vindication and score-settling. Some readers may suspect that my judgment was influenced by a generous reference to me at the end of the book. They are right, but not for the reason which they suppose. Earlier in Decline and Fall, I am the subject of two far from flattering mentions. Comparing the entries – and then comparing Mullin's comments on other extras in his cast – I realised he has published what he wrote down and that he wrote down what he actually thought at the time.

That notion was confirmed by the reproduction of instant judgments which turned out to be mistakes. On 9 June 2009, he was wise enough to describe John Bercow's chance of being elected Speaker as only "near fatal". But on 27 June 2005, he had described David Davis as the Tory Party's "leader in waiting". Davis was beaten so comprehensively by David Cameron in the leadership election that Mullin must have been tempted to omit his error. Its inclusion can only reinforce the conclusion that he takes his diary, and its integrity, seriously.

If he is to be applauded for publishing the mildly unpalatable truth, he should not be censured for doing the same with entries reporting his triumphs. But I read with initial embarrassment his account of a conversation which began with the solicitor general telling him, at the end of a debate on 90 days detention for terrorist suspects, that his "intervention was devastating". I read on, and discovered that Mullin's intervention was devastating in the sense that it damaged the cause which he supported. Part of this book's charm is the fact that its author, although the central character, is not its hero.

If the diary has a hero, it is Michael Foot, briefly mentioned but clearly adored. Gordon Brown, on the other hand, is a tragic figure – "obsessive, driven and somewhat lacking in the sense of humour department, but deep down a good and capable man". But, even as he praises the then chancellor for his address to the parliamentary Labour party, Mullin cannot resist giving him a gentle poke in the ribs. "He kept thanking us for what we were doing when, in truth, we were almost irrelevant to the success of his enterprise." The worst punishment Mullin inflicts is gentle ridicule.

In fact, he goes out of his way to protect feelings and reputations by withholding names which, had they been revealed, would have made the diaries more marketable. The Conservative MP who, during the expenses scandal, could be heard "relentlessly wailing 'I'm being vilified. My honour is at stake'," remains anonymous. So does the "colleague" who admitted in, June 2009, that her majority of 6,100 made her prospects in the forthcoming general election "iffy" at best. I would call Decline and Fall genial, if I did not fear that it would be regarded as a pejorative description by people who read political diaries to enjoy superannuated ministers being unpleasant about each other.

Mullin enjoys the advantage of writing naturally and well, and it is therefore all the more regrettable that he occasionally feels the need to provide a passage of "fine writing" – usually related to an incident that stirs his political emotions. After he passes "a Roma woman with what looks like a disabled child on her knee", he tells us: "Slowly, inexorably, inevitably, the chaos beyond our borders is beginning to lap at our comfortable little world." The unaffected passages about his elderly mother are far more moving.While absence of venom is Decline and Fall's virtue, its strength is its authenticity. Its title refers to the end of Mullin's political career, but it is really the story of the slow and painful death of the Labour government and the even more comprehensive demolition of a political innovation called New Labour. As I read the comments of doomed colleagues – "Gordon won't lead us into the election. Most people know he can't win. There is no malice. People are sad about it" – I felt I was back among the decent but self-destructive men and women of the parliamentary Labour party. Anyone who wants to know how life is for MPs who support a dying government and fear for their own survival should read Mullin's diary.

Roy Hattersley's The Great Outsider: The Biography of Lloyd George is published by Little Brown